1. Field of the Invention
The present invention relates to a control device, more particularly to a control device for suppression of residual vibration of a piezoelectric transducer.
2. Description of the Related Art
A parking sensor is usually implemented by using aluminum shell stuck with a piezoelectric transducer. When a driving signal is transmitted to the parking sensor, the piezoelectric transducer will transform electrical energy into mechanical force due to piezoelectricity so as to cause the aluminum shell to vibrate for generating an ultrasonic wave signal. After the emission of the ultrasonic wave signal, the parking sensor will transfer into the sensing mode and keep sensing an echo signal which was generated by reflection of the ultrasonic wave signal when bumping into an obstacle. By calculation of the time difference between the moment when the ultrasonic wave emit and the moment when the parking sensor receive the echo signal, one could estimate the distance between the sensor and the obstacle. If a vehicle equipped with the parking sensor is about to bump into an obstacle, the parking sensor could sense the distance between the vehicle and the obstacle and start to notify the vehicle driver when the distance difference reaches a set value. The warning sound of the notification could be in a beeping rhythm that corresponds to the distance between the vehicle and the obstacle in order to let the driver know that he is getting closer to the obstacle. Nevertheless, after emission of the ultrasonic wave signal, the piezoelectric transducer and the aluminum shell could not stop vibrating right away due to their elasticity and the residual vibration will damp away as the energy being drained to overcome frictional or other resistive forces. A recovery time which the parking sensor needs to damp the residual vibration away forms a blind range in sensing obstacles. Obstacles in the blind range could not be sensed since the echo signal reflected by it reaches the parking sensor while it is still in the residual vibration and the parking sensor could not distinguish the echo signal from the residual vibration. Therefore, the residual mechanical vibration of the piezoelectric transducer must be suppressed to reduce a minimum distance that could be estimated by the parking sensor.
Referring to FIG. 1 and FIG. 2, a control device for suppression of residual vibration of a piezoelectric transducer 1 includes a driving circuit 2, a residual control circuit 3, and a switch circuit 4. The control device could be operated either in a working mode or in a non-working mode. Under the working mode, the switch circuit 4 couples the driving circuit 2 electrically to the piezoelectric transducer 1, and the driving circuit 2 outputs a sinusoidal electric current as a driving signal for driving the piezoelectric transducer 1 to vibrate. Under the non-working mode, the switch circuit 4 couples the residual control circuit 3 electrically to the piezoelectric transducer 1, and the residual control circuit 3 suppresses residual vibration of the piezoelectric transducer 1. During residual vibration of the piezoelectric transducer 1, the piezoelectric transducer 1 outputs an electric current due to piezoelectricity. The electric current remains sinusoidal, yet has relatively low amplitude.
A conventional residual control circuit 3′ includes a direct current power source 31, and a switch unit 32. The direct current power source 31 supplies a direct current reference voltage. The switch unit 32 includes four switches 321 to 324. Under the non-working mode, when the electric current of the piezoelectric transducer 1 is positive (i.e., flow of the electric current from a first end of the piezoelectric transducer 1 toward a second end thereof, wherein the first end is connected to each of the switches 321, 322, and the second end is connected to each of the switches 323, 324), the switch unit 32 operates such that the switches 321, 323 conduct, and the direct current power source 31 provides a voltage equivalent to a positive direct current reference voltage for the piezoelectric transducer 1. When the electric current of the piezoelectric transducer 1 is negative (i.e., flow of the electric current from the second end of the piezoelectric transducer 1 toward the first end thereof), the switch unit 32 operates such that the switches 322, 324 conduct, and the direct current power source 31 provides a voltage equivalent to a negative direct current reference voltage for the piezoelectric transducer 1. In this way, the electric current outputted from the piezoelectric transducer 1 and the voltage applied thereto are in-phase, such that energy dissipation of the piezoelectric transducer 1 during each residual cycle is maximized for suppressing residual vibration of the piezoelectric transducer 1.
Even though the conventional residual control circuit 3′ may indeed suppress residual vibration of the piezoelectric transducer 1 in an initial stage of the non-working mode, when residual vibration thereof has been reduced to a very small level, energy of the direct current power source 31 may disturb convergence of vibration of the piezoelectric transducer 1 through the switch unit 32, and may even cause the piezoelectric transducer 1 to vibrate once again.
Furthermore, in “Velocity-Controlled Piezoelectric Switching Energy Harvesting Device” by Y. P. Liu et al., International Conference on Renewable Energies and Power Quality (ICREPQ), April, 2009, an energy harvesting device which harvests energy from a piezoelectric transducer is disclosed. The energy harvesting device uses super (ultra) capacitors to store harvested energy.